Method for producing silicon single crystal having no flaw

ABSTRACT

A method for producing a silicon ingot having no defect over a wide range of region with stability and good reproducibility, wherein when a silicon single crystal ( 11 ) is pulled up form a silicon melt ( 13 ), the shape of a solid-liquid interface ( 14 ) which a boundary between the silicon melt ( 13 ) and the silicon single crystal ( 11 ) and the temperature distribution on the side face ( 11   b ) of a single crystal under being pulled up are appropriately controlled.

TECHNICAL FIELD

This invention relates to a method for producing defect-free crystals(defect-free silicon single crystals), and to a silicon single crystalproduced by this method.

BACKGROUND ART

As semiconductor devices have been finer and more highly integrated inrecent years, there has been an increasing need for silicon wafers ofhigher quality. This also underscores the need to reduce crystal defectsthat occur in the course of producing silicon single crystals.

[Defects Contained in Single Crystals and Behavior thereof]

It has been understood that the following three types of crystal defectare commonly included in single crystals and related to deterioration ofthe performances of a device.

-   [1] Void defects thought to occur through the agglomeration of    vacancies-   [2] Oxidation induced stacking faults (OSF)-   [3] Dislocation clusters thought to occur through the agglomeration    of interstitial silicon

It is known that the way these defects occur varies as follows with thegrowth conditions.

-   [i] When the growth rate is high, single crystals have excessive    vacancies and therefore prone to the occurrence of only void    defects.-   [ii] If the growth rate is reduced, ring-like OSF occur around the    outer periphery of the crystals, and void defects are present on the    inside of the OSF portion.-   [iii] If the growth rate is further reduced, the radius of the    ring-like OSF decreases, dislocation clusters are produced on the    outer side of the ring-like OSF portion, and void defects are    present on the inner side of the OSF portion.

If the growth rate is reduced further yet, dislocation clusters areproduced throughout the crystal.

It is recognized that the above phenomena occur because a crystalchanges from a state of excess vacancy to a state of excess interstitialsilicon as the growth rate is lowered, and this change is understood tocommence from the outer peripheral of the crystal.

[Defect-Free Crystals (Defect-Free Silicon Single Crystals)]

As mentioned above, as device performances become more sophisticated,there is an increasing need to reduce the crystal defects that occur inthe course of producing silicon single crystals. With this in mind,there have been studies into the possibility of producing defect-freecrystals (perfect crystals), and a method for producing defect-freesilicon single crystals has been proposed in Japanese Laid-Open PatentApplication H8-330316 (hereinafter referred to as “Publication 1”).

Publication 1 states that a defect-free region was found where none ofthe above-mentioned three types of defect is present between thering-like OSF portion and the region where dislocation clusters occur.This defect-free region is understood to correspond to a transitionregion from an excess vacancy state to an excess interstitial siliconstate, and correspond to a neutral state that does not reach an excessamount at which any of the defects occur.

Publication 1 also states a proposal of a growth method by which thisneutral state is attained throughout an entire crystal. With thisproposed method, this neutral state can be attained throughout an entirecrystal by pulling up the crystal such that the ratio expressed by V/Gis kept within a range of 0.20 to 0.22 mm²/° C. min where V is thecrystal pull-up rate (mm/min)and G is the average temperature gradientwithin the crystal in the axial direction between the melting point ofsilicon and 1300° C. (° C./mm).

If G is constant in the radial direction, then when G=3.0° C./mm, forexample, the pull-up rate V should be controlled to 0.63±0.03 mm/min.This is not impossible in an industrial setting. Still, this only refersto the maximum permissible range, and is not actually practical. This isbecause if G varies, and is not uniform in the radial direction, thepermissible range becomes exceedingly small. For example, thepermissible range drops to zero once the change in G in the radialdirection reaches 10%. This means that slight decreases the uniformityof G make it essentially impossible to produce defect-free crystals(perfect crystals).

Moreover, since G is usually not constant in the radial direction, it isentirely conceivable that the change in G in the radial direction willindeed reach 10%. Because of this, with the method proposed inPublication 1, even if crystals are pulled up at the same pull-up rate,heater output and so forth, defect-free crystals will sometimes beobtained and sometimes not, meaning that the production of defect-freecrystals will be extremely unstable.

Furthermore, the following two problems are encountered with theproposal in Publication 1.

-   [1] G (the average temperature gradient within a crystal in the    axial direction) is hard to evaluate and difficult to predict.-   [2] G varies during pull-up.

Specifically, factors that cause G to vary during pull-up includechanges in the thermal balance resulting from a change in the length ofthe crystal, changes in the thermal balance resulting from a change inthe relative positions of the crucible and heater, and changes in theamount of melt, and it is difficult to ascertain and control these.

Also, whereas the growth rate V is a controllable parameter, theevaluation and prediction of G are very difficult, and changedynamically. Accordingly, a great deal of trial and error are inevitablein the specific implementation of this invention. That is, the relationbetween the specific settable parameters and the resulting G is unclear,so no specific means is known for achieving this end. Further, even thevalue of V/G, at which a neutral state is said to be obtained, can varyby two times depending on the research facility, and can even beconsidered as an uncertain value.

Japanese Laid-Open Patent Application H11-199386 (hereinafter referredto as “Publication 2”) acknowledges the industrial difficulty inproducing crystals that will in only this neutral state (method inPublication 1), and proposes a method for producing crystalssubstantially close to being free of defects, although permitting an OSFportion to remain in just an extremely small region at the crystalcenter. Publication 2 states that it was believed that the producingconditions under which this state is obtained are determined by V/G, andproposes the following as conditions for getting the entire crystal intothis region.

-   [a] The in-plane average G is less than 3° C./mm, and is less than    1.0° C./mm between Gedge and Gcenter. (Gedge is the average axial    temperature gradient on the crystal side face side, and Gcenter is    the average axial temperature gradient on the crystal center side.    V_(OSFclose) is the pull-up rate at which OSF rings disappear when    the pull-up rate is reduced.)-   [b] V is controlled to V_(OSFclose)±0.02 mm/min, and the average V    is controlled to V_(OSFclose)±0.01 mm/min.-   [c] The single crystal pull-up is performed with a magnetic field    applied, this magnetic field being a horizontal magnetic field, and    the magnetic field strength being 2000 G or greater.

Keeping the difference between Gedge and Gcenter to less than 1.0°C./mm, controlling V to V_(OSFclose)±0.02 mm/min, and controlling theaverage V to V_(OSFclose)±0.01 mm/min, as in Publication 2, are withinthe ranges proposed in Publication 1, and what is presented as newinformation is that it is easier to obtain defect-free crystals whenthere is a low temperature gradient, with the in-plane average G beingless than 3° C./mm, and that the application of a magnetic field iseffective.

Japanese Laid-Open Patent Application H11-199387 (hereinafter referredto as “Publication 3”) proposes a method for producing defect-freecrystals containing no OSF portion. Publication 3 states that there aretwo types of neutral region, and noting that there is a defect-freeregion in which vacancies are predominant and a defect-free region inwhich interstitial silicon is predominant, the proposal was made of amethod for producing defect-free crystals in which interstitial siliconis predominant.

As to the conditions for pulling up defect-free crystals, the in-planechange in G is adjusted so that (Gmax−Gmin)/Gmin will be less than 20%.This is also within the proposed range given in Publication 1, and nospecific method is disclosed. The value of G given in Publication 3 isdetermined by heat transfer analysis (FEMAG), and not only is theabsolute value of G not known, it is not even certain whether thedistribution trend in the radial direction itself corresponds to actualcrystals.

Japanese Laid-Open Patent Application H11-79889 (hereinafter referred toas “Publication 4”) proposes a method for producing crystals so thatjust the neutral state is produced. This method involves flating of theshape at the solid-liquid interface, and it is proposed that the pullingbe such that the height of the solid-liquid interface will be no morethan ±5 mm with respect to the average value. In this case, G isuniform, and Gedge and Gcenter can be kept under 0.5° C./mm. Applying amagnetic field is an effect way to obtain a flat solid-liquid interfaceshape such as this, and a horizontal magnetic field of 2000 Gauss orgreater is said to be best.

The new finding of this proposal is that the shape of the solid-liquidinterface is identified as a factor. The given G value, however, isfound by heat transfer analysis (FEMAG), just as in Publication 3.However, just because the solid-liquid interface is flat does notautomatically mean that G will be uniform, so not only is the absolutevalue of G not known, it is not even certain whether the distributiontrend in the radial direction itself corresponds to actual crystals.

As discussed above, with prior proposals, crystals free of defects couldbe obtained if the growth rate V and the axial temperature gradient Gnear the solid-liquid interface were appropriately controlled. However,as described above, in addition to the fact that G is dynamic, changingfrom moment to moment while a crystal is being pulled up, it is alsoextremely difficult to evaluate or predict this value accurately.Actually, even the value of V/G, at which a neutral state is said to beobtained, can vary by two times depending on the research facility, andcan even be considered an uncertain value.

Thus, whereas the growth rate V is a controllable parameter, theevaluation and prediction of G are very difficult, and changedynamically. Therefore, a great deal of trial and error were inevitablein the specific implementation of the inventions according to theabove-mentioned conventional technology.

Also, the relation between the specific settable parameters and theresulting G is uncertain, so no specific means is known for determiningthe proper G in all publications related to the aforementioned priortechnology.

DISCLOSURE OF THE INVENTION

The present invention was made in light of the above problems, and it isan object thereof to ascertain the specific conditions necessary toobtain defect-free crystals stably and with good reproducibility, and toprovide a method for producing a silicon ingot having a broaddefect-free region stably and with good reproducibility.

In order to achieve the stated object, the present invention ischaracterized in that defect-free crystals are produced stably and withgood reproducibility by suitably adjusting the relation between theshape of the solid-liquid interface and the temperature distribution onthe side face of a single crystal being pulled up.

[Basic Concept of the Invention, and Effect of the Solid-LiquidInterface Shape on the Formation of the Defect-Free Crystals]

The fact discovered by the inventors of the present application that theshape of the solid-liquid interface is closely related to the formationof defect-free crystals, and that this “shape of the solid-liquidinterface” is a parameter that can actually be controlled, contributedgreatly to the inventors' being able to make the present invention.

The phrase “shape of the solid-liquid interface” as used here is theportion that becomes the interface when a silicon melt solidifies andforms silicon single crystals. As shown in FIG. 25, the solid-liquidinterface 14 can be defined as the boundary between the silicon singlecrystal 11 and the silicon melt 13. This solid-liquid interface 14 mayprotrude upward (FIG. 25A) or downward (FIG. 25B). Depending on thecase, the boundary may also be flat (FIG. 25C), or it may be wavy (FIG.25D).

The fact that the shape of this solid-liquid interface 14 is closelyrelated to the convection of the silicon melt 13 is another discovery bythe inventors, and since it is possible to control the convection of thesilicon melt 13, as a result it is also possible to control the shape ofthe solid-liquid interface 14.

The following is a more detailed description of the information newlyobtained by the inventors. The reason it is difficult to evaluate andpredict G (the average temperature gradient within a crystal in theaxial direction) is that G is greatly affected by the shape of thesolid-liquid interface. Because the shape of the solid-liquid interfaceis governed in large measure by the melt convection, the distribution ofG cannot be accurately predicted unless the prediction is correct alsoon the melt convection. Thus, the precision at which G can be predictedby heat transfer simulation that does not take melt convection intoaccount will naturally be low.

For example, results of evaluating G that do not include the convectioneffect cannot be used in the setting of conditions that requirehigh-precision evaluation, as in adjusting the V/G ratio to between 0.20and 0.22 mm²/° C.min. However, a major obstacle in performing thissetting is that there is not as yet a technique for predicting a meltconvection with a crystal pull-up apparatus wherein the size is on thecurrent industrial level. With the present invention, it is possible toconduct an evaluation of G that includes the convection effect, andcontrol the V/G ratio at extremely high precision, by establishing atechnique for predicting the effect of a melt convection.

Also, G is dynamic, changing from moment to moment, and examples offactors that can change G during pull-up include changes in the thermalbalance resulting from a change in the length of the crystal, changes inthe thermal balance resulting from a change in the relative positions ofthe crucible and heater, and changes in the amount of melt. Theinventors have hit upon the conclusion that the reason G fluctuates as aresult of these changes is that these changes cause the convection of asilicon melt to change, and the shape of the solid-liquid interface tochange as well.

Therefore, with the present invention, basically the shape of thesolid-liquid interface is adjusted by controlling parameters related tothe convection of the silicon melt 13, whereby defect-free crystals areobtained stably and exactly.

To this end, as shown in FIG. 26, the hot zone must be controlled sothat the “height of the solid-liquid interface” defined as the height hof the solid-liquid interface 14 at the center line 11 a of a crystal 11(that is, the height h of the solid-liquid interface at the crystalcenter) will be correlated with the temperature gradient in the pullingdirection on the side face 11 b of the crystal 11. (This will later bedescribed in further detailed manner.)

Incidentally, as shown in FIG. 27, the basic structure of the hot zonein a standard CZ furnace includes a crucible 21 that holds the siliconmelt 13 and self-rotates, a heater 22 that heats this crucible 21, aheat shield 23 that surrounds the single crystal 11 being pulled up fromthe silicon melt 13 while rotating, and adjusts the amount of heatradiated to this single crystal 11, side face temperature adjustingmeans 24 for adjusting the temperature of the side face 11 b of thesingle crystal 11, and a solenoid 26 for applying a magnetic field tothe silicon melt 13.

The heat shield 23 is generally made from a carbon material, and adjuststhe temperature of the side face 11 b of the single crystal 11 byblocking heat radiated from the silicon melt 13 and so forth, while theside face temperature adjusting means 24, which is disposed surroundingthe single crystal 11 just as is the heat shield 23, is made of amaterial that actively absorbs heat or performs heating, such as acooler or a heater. Also, as shown in FIG. 27, the heater 22 preferablycomprises a side heater 22 a and a bottom heater 22 b.

The control and adjustment of the convection of the silicon melt forimplementing the present invention, and in turn the control andadjustment of the height of the solid-liquid interface (h in FIG. 26),can be performed by adjusting the rotational speed of the crucible perunit of time, adjusting the rotational speed of the crystal per unit oftime, and applying a magnetic field and adjusting the strength of thisapplied field.

In general, the height of the solid-liquid interface will increasewhether the rotational speed of the crucible per unit of time isincreased, the rotational speed of the crystal per unit of time isincreased, or a magnetic field is applied. In order to producedefect-free crystals according to the present invention, the temperaturegradient of the side face 11 b of the single crystal 11 must beincreased as the height of the solid-liquid interface rises.

[Conventional Recognition of the Effect of the Solid-Liquid InterfaceShape and Melt Convection]

The control of the shape of the solid-liquid interface and the controlof the convection of the silicon melt have not been touched uponwhatsoever in the above-mentioned conventional technology. The followingis all there is on this topic.

First, in Publications 2 and 3, G (the average temperature gradientwithin a crystal in the axial direction) is evaluated by FEMAG (F.Dupret, P. Nicodeme, Y. Ryckmans, P. Wouters, and M. J. Crochet, Int. J.Heat Mass Transfer, 33, 1849 (1990)). This evaluation procedure merelyinvolves evaluating the environment of thermal radiation, conduction andtransfer within the hot zone, and the effect of the melt convection isnot considered. However, as described above, since G is actuallygoverned in large measure by the melt flow state, not only is the Gindicated by this procedure not the absolute value, but it is not evencertain whether the distribution trend in the radial direction itselfcorresponds to actual crystals.

Therefore, the distribution of the actual G (average temperaturegradient within a crystal in the axial direction) in an actual crystalused in their experiment and the G claimed in the Claims section canhardly be said to match up in a perfect correlation, and if taken to theextreme, it might even be said that the two are not related in any wayat all.

This same situation applies to the inventions according to Publications1 and 4, in which the effect of melt convection and solid-liquidinterface shape is either not taken into account at all, orsubstantially not considered, so when G is tracked precisely, it canhardly be said that the actual G distribution matches up perfectly withthe G indicated in the Claims section. Consequently, when the inventionaccording to Publication 1 is implemented, for example, defect-freecrystals cannot be obtained stably even after adjustment to the V andV/G indicated in the Claims.

In this respect, since Publication 1, for instance, gives essentially nothought to the considerable effect that the solid-liquid interface shapehas on G, the examples of producing defect-free crystals given thereinare producing examples for whatever shape the solid-liquid interfacehappened to have, and do not indicate the optimal conditions. Also,since the production was performed in a state in which the solid-liquidinterface shape was not fixed, this means that the examples ofsuccessful production of defect-free crystals also included aconsiderable number of accidental factors.

Incidentally, a method for producing crystals that results in only aneutral state has been proposed in Japanese Laid-Open Patent ApplicationH10-330713 (invention previously submitted by this company). Theconditions therein are that V/G is from 0.16 to 0.18 mm²/° C.min and theratio of Gedge/Gcenter is 1.10, but at the time the patent applicationwas submitted for this invention, melt convection had not yet been giventhat much thought.

[Principle of the Present Invention]

Research on the part of the inventors has revealed that the factorswhich determine the axial temperature gradient G of a crystal are theshape of the solid-liquid interface and the temperature distribution onthe side face of the crystal. Since the temperature at the solid-liquidinterface is the melting point, if these two factors are determined,then the boundary conditions for determining the temperature of thecrystal will be determined, and the steady state temperaturedistribution inside the crystal will also be uniquely determined. Inview of this, the occurrence of crystal defects is controlled in thepresent invention by using these two factors as operating parameters.

<Shape of the Solid-Liquid Interface>

The shape of the solid-liquid interface can be controlled as desired bycontrolling the parameters that vary the melt convection. Thetemperature distribution on the outer side face of the crystal can alsobe controlled as desired through the structure of the hot zone.

In view of this, the inventors sought the conditions under whichdefect-free crystals would be readily obtained by controlling the shapeof the solid-liquid interface and the temperature distribution on theouter side face of the crystal.

First they examined the relation between G and the shape of thesolid-liquid interface by heat transfer analysis. FIG. 1 illustrates anexample of the solid-liquid interface shape near the growth rate atwhich the OSF rings disappear. As shown in FIG. 1, at such a low growthrate, the location of the solid-liquid interface generally becomesgull-shaped (along with FIG. 1, also see the solid-liquid interface 14in FIG. 26B), so that the interface is convex on the melt side, butdepending on the state of the melt convection, it can also be convex onthe crystal side.

Next, as shown in FIG. 2, the solid-liquid interface shape is patternedon the basis of the shape in FIG. 1, and the virtual interface shape isset such that the difference between the height of the solid-liquidinterface at the crystal center and the solid-liquid interface height atthe outer periphery of the crystal (hereinafter referred to as the“height of the solid-liquid interface”) is varied in 5 mm intervals from−20 mm to +20 mm. (As described above, illustrated schematically, asshown in FIG. 26, the “height of the solid-liquid interface” is definedas the height h of the solid-liquid interface 14 at the center line 11 aof the crystal 11.)

<Temperature Distribution on Crystal Side Face>

For the temperature distribution on the crystal side face, thetemperature distribution shown in FIG. 3 was set by patterning thecrystal side face temperature distribution obtained from global heattransfer analysis. (“Crystal side face” refers to the side face 11 b ofthe crystal 11 shown in FIG. 26.) The set temperature distribution T(X)is given by the following Formula 1.T(X)K=1685K−1.78X+aX(X−400)²  (1)

where X is the distance (mm) from the solid-liquid interface, and T(X)is the absolute temperature on the crystal side face at the distance Xmmfrom the solid-liquid interface.

Assuming that the temperature gradient at the solid-liquid interface inthis formula be G₀=−(dT(X)/dX)_(x=0), then G₀=1.78+1.6×10⁵a is obtained,where a was set to give the value of G₀ for the crystal side face in0.5° C./mm intervals from 1.5 to 5.0° C./mm, the length of the crystalwas assumed to be 400 mm (cylindrical), and the crystal diameter to be210 mm.

FIG. 4 is a graph of the contour lines of G at the crystal center whenthe horizontal axis is the height of the solid-liquid interface, and thevertical axis is the G of the crystal side face (the temperaturegradient in the crystal pull-up direction; hereinafter G will be used inthis meaning in describing the present invention).

<Height of the Solid-Liquid Interface and G at the Crystal Side Face andthe Crystal Center>

It can be seen that G at the crystal center (the temperature gradient inthe crystal pull-up direction; the same applies hereinafter) variesgreatly with the height of the solid-liquid interface. It must be notedhere that G refers to the temperature gradient from the melting point upto 1350° C. at both the crystal side face and the crystal center, and isnot G₀.

FIG. 5 shows the ratio between the temperature gradient G on the outerside face of the crystal and the temperature gradient at the crystalcenter, calculated on the basis of the above calculation results. Asshown in FIG. 5, it can be seen that the conditions under which theratio between the temperature gradient G on the outer side face of thecrystal and the temperature gradient at the crystal center is close to 1and G is uniform vary with the shape of the solid-liquid interface.Furthermore, it can also be seen that to obtain a uniform G, the greateris the height of the solid-liquid interface, the greater the temperaturegradient must be on the side face.

It is stated in Publication 4 that G is uniform when the shape of thesolid-liquid interface is flat, but if we refer to these calculationresults (FIG. 5) we see that this condition only holds true when G onthe crystal side face is small. Also, reference to these calculationresults reveals that when the height h of the solid-liquid interface isgreat, a uniform G is obtained even with a high temperature gradient. Inregard to this, Publication 2 states that neutral crystals are easier toobtain at a low temperature gradient at which the in-plane average G isless than 3° C./mm. However, it can be seen that as long as the heightof the interface is set high, a uniform G will be obtained even at ahigh temperature gradient.

<Conditions under which Defect-Free Crystals are Obtained>

The inventors turned their attention to this relationship and searchedfor conditions that would readily yield defect-free crystals. The heightof the solid-liquid interface (h in FIG. 26) was adjusted by means ofthe rotational speed of the crucible per unit of time, the rotationalspeed of the crystal per unit of time, whether or not a magnetic fieldwas applied, and the strength of this magnetic field. The temperaturegradient on the crystal side face (11 b in FIG. 26) was adjusted bymeans of the height of the heat shield (such as the heat shield 23 inFIG. 27) surrounding the crystal from the liquid surface.

The height of the solid-liquid interface was evaluated by slicing apulled-up crystal vertically including the crystal axis to obtain asheet-form sample, and then observing the growth striation by X-raytopography.

The temperature gradient on the crystal side face was determined byglobal heat transfer analysis. It was mentioned in the previous sectionthat the evaluation of G by global heat transfer analysis has poorprecision because the effect of the melt convection cannot be accuratelyevaluated. However, this is because the location of the solid-liquidinterface varies with the melt convection. In principle, thesolid-liquid interface height at the ends of the crystal side face isalmost unchanged by convection. Therefore, the reliability of this valueis better than that of G in the crystal interior.

Under pull-up conditions including a crystal diameter of 210 mm and acrucible diameter of 22 inches, the distribution of defect type wasexamined to see how it varies with the combination of conditions of G onthe crystal side face and the solid-liquid interface height. Crystalswere grown while the growth rate was steadily lowered. The distributionof defect type at various locations in the crystals was evaluated bySecco etching and X-ray topography after heat treatment, and by copperdecoration.

TABLE 1 Solid-liquid Temperature gradient Magnetic field Sampleinterface height on crystal side face application Crystal in −5.7 mm2.1° C./mm none FIG. 6 Crystal in +7.0 mm 2.7° C./mm horizontal magneticFIG. 7 field 2000 Gauss Crystal in +14.7 mm  2.82° C./mm  horizontalmagnetic FIG. 8 field 3000 Gauss

FIGS. 6, 7, and 8 show examples of evaluation results for the abovethree crystals. FIGS. 6, 7, and 8 illustrate the regions in which eachtype of defect is present, where the horizontal axis is the growth rate,and the vertical axis is the location in the radial direction of thecrystal.

The growth rate range in which defect-free crystals can be obtained is,of course, between the minimum rate Vosf,min in the radial direction ofthe transition rate between the OSF region and the defect-free region,and the maximum rate Vdis,max in the radial direction of the transitionrate between the defect-free region and the dislocation cluster region.Vosf,min and Vdis,max as defined here are indicated by the broken linesin FIGS. 7 and 8.

Here, if ΔV=Vosfmin−Vdis,max, then defect-free crystals will be obtainedonly when ΔV is positive. In addition, when ΔV is defined in this way,it is also the permissible range for pull-up rate, and the larger thisvalue is, the better suited it will be to industrial production.Furthermore, the larger is the average value V of Vosf,min and Vdis,max,the more quickly the single crystals can be pulled up, so the larger isthe average value V, the better is the productivity.

FIG. 9 shows contour lines for ΔV, where the horizontal axis is theheight of the solid-liquid interface, and the vertical axis is G on thecrystal side face. Here, the growth of defect-free crystals is possibleif ΔV is positive, that is, in terms of FIG. 9, only within the rangeindicated by the hatching in FIG. 9. As for the industriallycontrollable range, the range indicated by the cross-hatching in FIG. 9(ΔV>0.01 mm/min) is preferable.

Meanwhile, FIG. 10 shows the contour lines for the average value V ofVosf,min and Vdis,max, where the horizontal axis is the height of thesolid-liquid interface, and the vertical axis is G on the crystal sideface. It can be seen from FIGS. 9 and 10 that the higher the location ofthe solid-liquid interface, and the higher the temperature gradient onthe crystal side face (hereinafter referred to as high temperaturegradient conditions), the faster the defect-free crystals can be pulledup.

<Discrepancies Between Conventional Findings and Findings of theInventors>

Findings according to prior inventions have held that defect-freecrystals will not be obtained unless G is uniform. With this in mind, wecompared the range over which defect-free crystals are obtained in FIG.9 with the uniformity of the temperature gradient in FIG. 5. Thisrevealed that the conditions for good uniformity do not correspondperfectly to the range over which defect-free crystals are obtained. Inother words, under conditions comprising a low temperature gradient onthe crystal side face (hereinafter referred to simply as low temperaturegradient conditions), defect-free crystals will be obtained even if G isnot very uniform at all. This finding is in contrast to the findingsaccording to prior inventions.

Furthermore, past findings have held that there exists a certain V/Gratio at which defect-free crystals are formed. This has been treated asif it were a matter of course because according to the Voronkov theory(V. V. Voronkov, J. Crystal Growth, Vol. 59, p. 625, 1982), the growthconditions in which there is a neutral state, with the vacancy andinterstitial silicon concentrations being equal, are determined by V/G.

On the other hand, though, there is an extraordinary amount of variancein the reported values for the V/G critical value among various researchfacilities. This is probably because none of these reports looked at theeffect that the shape of the solid-liquid interface has on G.

In view of this, we found the G value that takes into account the shapeeffect of the solid-liquid interface, for the examples shown in FIGS. 6,7, and 8, and conducted an evaluation using the V/G for the defectregion. In this G evaluation method, a measured solid-liquid interfaceshape was used in the calculations, and the temperature distribution onthe crystal side face was found by heat transfer calculation by settingthe distribution found by global heat transfer analysis.

These results are given in FIGS. 11, 12, and 13. It can be seen from thegraphs that the V/G at which a defect-free region occurs varies greatlywith the height of the solid-liquid interface. It can also be seen fromFIGS. 11, 12, and 13 that the V/G value at which defect-free crystalsare obtained varies with the solid-liquid interface shape.

The V/G values reported in Publication 1 corresponded to the values inFIG. 11 in the case of the crystal in FIG. 6, in which the solid-liquidinterface was lowered down. Since the width of V passing through thewafer plane in FIG. 6 is extremely narrow, it can be seen that it wouldbe extremely difficult to control all the wafer planes so as to bedefect-free. This underscores the fact that it would be quite difficultto produce defect-free crystals on the basis of the report inPublication 1.

<Voronkov Theory and the Present Invention>

While it cannot be predicted from the Voronkov theory that the criticalV/G varies with the shape of the solid-liquid interface, the theory doescorroborate this phenomenon somewhat. We will now give a briefdescription of how the Voronkov theory corroborates that the criticalV/G varies in relation to the shape of the solid-liquid interface.

According to the Voronkov theory, at the solid-liquid interface ofsilicon, vacancies and interstitial silicon are in about the sameconcentration as point defects in thermal equilibrium, with vacanciesthought to be present in slightly greater quantity. Interstitial siliconis said to have a larger diffusion coefficient at high temperatures.Since vacancies and interstitial silicon rapidly lower theconcentrations of each other through a pair annihilation reaction, aconcentration gradient is produced, and inflow through diffusion occursat the solid-liquid interface. At this point, the interstitial silicon,which has a larger diffusion coefficient, enters in a larger quantity,so under low-speed pull-up conditions in which much diffusion time isprovided, an inversion occurs in the concentration relationship, withinterstitial silicon becoming dominant. On the other hand, underhigh-speed pull-up, not much diffusion time is provided, and only a pairannihilation reaction occurs, so the vacancies that were dominant in thefirst place remain dominant in this model.

Voronkov arrived at the fact that the growth conditions which result ina neutral state are described by a specific V/G on the premise of theone-dimensional diffusion of point defects. Specifically, thiscorresponds to a scenario in which the solid-liquid interface is flatand the temperature distribution is uniform in the radial direction.Actual crystals, though, can not described by a one-dimensional model.According to the thinking of Voronkov, since the concentration gradientof point defects occurs in the temperature gradient direction, a strongconcentration gradient is generated perpendicular to the solid-liquidinterface.

In other words, when the solid-liquid interface is not flat, thediffusion of interstitial silicon occurs not in the crystal axialdirection, but perpendicular to the solid-liquid interface, so it may bethat the shape of the solid-liquid interface also has two-dimensionaldiffusion effects, such as the concentration of interstitial silicon onthe crystal center side, or diffusion outward from the crystal.Therefore, our experimental results, which indicate that “the criticalV/G at which a neutral state is obtained is dependent on the shape ofthe solid-liquid interface,” can be qualitatively corroborated byVoronkov's theory of interstitial silicon diffusion.

[Disclosure of the Invention]

The present invention, which was conceived in light of the abovesituation, is specifically described as follows.

-   (1) A method for producing defect-free crystals, wherein the shape    of the solid-liquid interface between a silicon melt and a silicon    single crystal is taken into account in the pulling of a silicon    single crystal from a silicon melt by CZ (Czochralski) method.

In the context of the essence of the present invention, “A method forproducing defect-free crystals, wherein the height of the solid-liquidinterface between a silicon melt and a silicon single crystal, which isthe height of the solid-liquid interface at the crystal center, is takeninto account in the pulling of a silicon single crystal from a siliconmelt by CZ method” and “A method for producing defect-free crystals,wherein the convection of a silicon melt is taken into account in thepulling of a silicon single crystal from the silicon melt by CZ method”can be viewed as being substantially the same as (1) above.

-   (2) A method for producing a silicon ingot by pulling up the silicon    single crystal from the silicon melt by CZ method by adjusting the    shape of the solid-liquid interface, which is the boundary between a    silicon melt and a silicon single crystal, and the temperature    distribution on the side face of the silicon single crystal, thereby    producing a silicon ingot including a defect-free region.-   (3) The method according to (2), wherein the shape of the    solid-liquid interface is adjusted by adjusting the height of the    solid-liquid interface at the crystal center, and the temperature    distribution on the side face of the silicon single crystal is    adjusted by adjusting the temperature gradient in the pulling    direction in the periphery of the crystal.-   (4) The method according to (3), wherein the height of the    solid-liquid interface at the crystal center is 10 mm or higher.-   (5) The method according to (3), wherein the adjustment of the    height of the solid-liquid interface at the crystal center is    accomplished by one or more ways selected from the group consisting    of adjusting the strength of the magnetic field applied to the    silicon melt, adjusting the rotational speed per unit of time of the    crucible containing the silicon melt, and adjusting the rotational    speed per unit of time of the silicon single crystal.-   (6) The method according to (5), wherein the strength of the    magnetic field is 2500 Gauss or greater.-   (7) A method for increasing the production efficiency of defect-free    crystals by setting the height of the crystal center portion of the    solid-liquid interface between a silicon melt and a silicon single    crystal to 10 mm or greater and increasing the rate at which the    silicon single crystal is pulled up in the pulling of a silicon    single crystal from a silicon melt by CZ method. “The height of the    crystal center portion of the solid-liquid interface” is defined the    same as “the height of the solid-liquid interface at the crystal    center.”-   (8) An apparatus for producing a silicon ingot by pulling up a    silicon single crystal from a silicon melt by CZ method, wherein the    silicon single crystal is pulled up such that the height of the    solid-liquid interface at the crystal center portion of the    solid-liquid interface between a silicon melt and a silicon single    crystal, and the temperature gradient on the side face of the    silicon single crystal fall within the region indicated by the    hatched region in FIG. 9.

However, this apparatus results in excluding the situations where asilicon single crystal is pulled up under any of the following singlecrystal pull-up conditions I to III:

-   (I) the ratio expressed by V/G is controlled to be from 0.20 to 0.22    mm²/° C.min, where V is the crystal pull-up rate (mm/min) and G is    the average temperature gradient within the crystal in the axial    direction between the melting point of silicon and 1300° C. (°    C./mm),-   (II) the in-plane average G is less than 3° C./mm, less than 1.0°    C./mm between Gedge and Gcenter, V is controlled to    V_(OSFclose)±0.02 mm/min, the average V is controlled to    V_(OSFclose)±0.01 mm/min, and in a horizontal magnetic field the    applied magnetic field strength is 2000 G or greater(Gedge is the    average axial temperature gradient on the crystal side face side;    Gcenter is the average axial temperature gradient on the crystal    center side; and V_(OSFclose) is the pull-up rate at which OSF rings    disappear when the pull-up rate is reduced.)-   (III) the V/G value between the crystal center location and a    location up to the crystal outer periphery is from 0.16 to 0.18    mm²/° C. ·min, and Gouter/Gcenter ≦1.10, where V is the crystal    pull-up rate (mm/min), G is the average temperature gradient within    the crystal in the axial direction between the melting point of    silicon and 1350° C. (° C./mm), Gouter is the G value on the outer    side face of the crystal, and Gcenter is the Gvalue at the crystal    center.

In regard to the region indicated by the hatching in FIG. 9, it isactually preferable if the height of the solid-liquid interface and thetemperature gradient on the crystal side face are controlled so as to bewithin the region indicated by the cross-hatching in FIG. 9. Whenproduction efficiency of a silicon ingot or a silicon wafer isconsidered, the data in FIG. 10 are also taken into account. In atypical embodiment, the data in FIG. 9 and the data in FIG. 10 are bothstored in the memory of the silicon ingot producing apparatus, and theapparatus reads out this data as needed to set the pull-up conditions.

-   (9) A defect-free crystal produced by pulling at a rate of 0.40    mm/min or greater (and especially at a rate of 0.45 mm/min or    greater). The defect-free crystals are preferably low-oxygen    defect-free crystals, and as shown in an embodiment given below, the    pull-up rate can be set between 0.56 and 0.49 mm/min.-   (10) A silicon ingot with a diameter of 200 mm or greater, and 55%    (ratio of length to overall silicon ingot length) or greater (and    preferably 70% or greater) of which is a region in which the entire    wafer plane is defect-free. Producing a large-diameter silicon ingot    having such a broad defect-free region is novel because it was    impossible with the methods for producing defect-free crystals    according to conventional technology, and this is also encompassed    by the scope of the present invention. The phrase “a region in which    the entire wafer plane is defect-free” refers to a region in which    an entire plane that is cut from an ingot is free of defects.-   (11) A group of silicon ingots wherein five or more (and preferably    ten or more) silicon ingots are produced continuously, and made up    solely of silicon ingots with a diameter of 200 mm or greater, and    50% (ratio of length to overall silicon ingot length)or greater    (more preferably 55% or greater) of which is a region in which the    entire wafer plane is defect-free.

With the methods for producing defect-free crystals according toconventional technology, because it was so difficult to producedefect-free crystals stably, large-diameter silicon ingots 55% orgreater of which is a region in which the entire wafer plane isdefect-free could not be produced stably (such as five or more ingotscontinuously). However, this is possible with the present invention.

-   (12) A silicon ingot with an oxygen concentration of 24 ppma or less    and a diameter of 200 mm or greater, and 40% (ratio of length to    overall silicon ingot length)or greater of which is a region in    which the entire wafer plane is defect-free.

The following is another method for producing a silicon ingotcorresponding to the silicon ingot producing apparatus described in (8)above.

-   (13) A method for producing a silicon ingot by pulling up a silicon    single crystal from a silicon melt by CZ method, wherein the silicon    single crystal is pulled up such that the height of the solid-liquid    interface at the crystal center portion of the solid-liquid    interface between a silicon melt and a silicon single crystal, and    the temperature gradient on the side face of the silicon single    crystal fall within the region indicated by the hatched region in    FIG. 9.

It can also be seen that one aspect of the present invention is anextremely effective method for finding the optimal conditions forproducing defect-free crystals. Therefore, the present inventionencompasses in its scope the following as a concept thereof, and theacts of analyzing the shape of a solid-liquid interface by using anexisting electrothermal analysis system or the like, and seeking theoptimal conditions for producing defect-free crystals are allencompassed in the scope of the present invention.

-   (14) A method for finding the optimal conditions for producing    defect-free crystals by monitoring the shape of the solid-liquid    interface between a silicon melt and a silicon single crystal in the    pulling of a silicon single crystal up from a silicon melt by CZ    method.

<(9) to (12) and Conventional Technology>

As to an ingot in which entire wafer planes are present as a defect-freeregion, M. Hourai, H. Nishikawa, T. Tanaka, S. Umeno, E. Asayama, T.Nomachi, and G. Kellty have disclosed in “Semiconductor Silicon,”Electrochemical Society Proceedings PV 98-1, 1998, p. 453 (hereinafterreferred to as Publication 5) an ingot in which 73% (the value obtainedjudging from the photograph provided) of the entire length of a 6-inch(150-mm) crystal is such a defect-free region. Also, J. G. Park, G. S.Lee, J. M. Park, S. M. Chou, and H. K. Chung have reported in “Defect inSilicon III,” Electrochemical Society Proceedings PV 99-1, 1999, p. 324(hereinafter referred to as Publication 6) that, regarding an ingot of adiameter of 200 mm, 600 mm (that is, 50% of the total length) out of theentire 1200 mm length of a crystalis defect-free.

Therefore, it is clear that the “silicon ingot with a diameter of 200 mmor greater and 55% (ratio of length to overall silicon ingot length) orgreater (and preferably 70% or greater) of which is a region in whichthe entire wafer plane is defect-free” mentioned in (10) above is novel.

Also, even if a silicon ingot with a diameter of 200 mm or greater and50% or greater of which is a region in which the entire wafer plane isdefect-free can be produced with the method in Publication 6, such aningot cannot be produced stably, so it is also clear that the “group offive or more (and preferably ten or more) silicon ingots producedcontinuously, said silicon ingot group being made up solely of siliconingots with a diameter of 200 mm or greater and 50% (ratio of length tooverall silicon ingot length)or greater (more preferably at 55% orgreater) of which is a region in which the entire wafer plane isdefect-free” mentioned in (11) above is novel.

In the above-mentioned Publications 5 and 6, there is no disclosure madeof an oxygen concentration of 24 ppma or less, so the “silicon ingotwith an oxygen concentration of 24 ppma or less and a diameter of 200 mmor greater, and 40% (ratio of length to overall silicon ingot length) orgreater of which is a region in which the entire wafer plane isdefect-free” mentioned in (12) above is also clearly novel.

Incidentally, no mention of the crystal pull-up rate is made in theabove-mentioned Publication 6, while Publication 5 states that a crystalis pulled up at 0.40 mm/min. Nevertheless, the crystal pulled up at 0.40mm/min in Publication 5 is 6 inches (150 mm), not 200 mm. Also,Publication 4 reports that a defect-free state was formed at a crystalpull-up rate of 0.55 mm/min. However, when the pull-up is performed atthis rate, only part of the crystal is in a defect-free state, and aningot in which entire wafer planes are present as a defect-free regionwas not produced at this pull-up rate. The same holds true for thereport by M. Iida, W. Kusaki, M. Tamatsuka, E. Iino, M. Kimura, and S.Muraoka (“Defect in Silicon III,” Electrochemical Society Proceedings PV99-1, 1999, p. 499 (Publication 7)) that a defect-free state was formedat a crystal pull-up rate of 0.53 mm/min.

Therefore, it is also clear that the “defect-free crystal produced bypulling at a rate of 0.40 mm/min or greater (and especially at a rate of0.45 mm/min or greater)” mentioned in (9) above is novel.

The term “defect-free crystal (perfect crystal)” means a crystal inwhich neither void defects, nor oxidation induced stacking faults (OSF),or dislocation clusters are present. The term “defect-free region(perfect crystal region)” or “region free of defects” means a region ina crystal in which neither void defects, oxidation induced stackingfaults (OSF), nor dislocation clusters are present.

The solid-liquid interface shape can be adjusted by adjusting therotational speed of the crystal per unit of time, adjusting therotational speed of the crucible per unit of time, adjusting magneticfield strength and direction, adjusting the bottom heater output, or acombination of these, while the axial temperature gradient (G) on thecrystal side face can be adjusted by adjusting the distance between theheat shield and the silicon melt, installing a cooler or heater thatsurrounds the crystals being pulled up, adjusting the bottom heateroutput, or a combination of these.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of the shape of a solid-liquid interfaceat a growth rate in the neighborhood of the growth rate at which the OSFrings disappear;

FIG. 2 illustrates a set of interface shapes preset for calculation;

FIG. 3 is a graph of the temperature distribution on the crystal sideface;

FIG. 4 is a graph of the contour lines for the temperature gradient G atthe crystal center, where the horizontal axis is the height of thesolid-liquid interface and the vertical axis is G on the crystal sideface;

FIG. 5 is a graph of the contour lines for the ratio (Gedge/Gcenter)between the temperature gradient G on the outer side face of the crystaland the temperature gradient at the crystal center, where the horizontalaxis is the height of the solid-liquid interface and the vertical axisis G on the crystal side face;

FIG. 6 is a graph of the relation between the pull-up rate and theregion in which each type of defect is present in the radial direction;

FIG. 7 is a graph of the relation between the pull-up rate and theregion in which each type of defect is present in the radial direction;

FIG. 8 is a graph of the relation between the pull-up rate and theregion in which each type of defect is present in the radial direction;

FIG. 9 is a graph of the contour lines for Vosf,min−Vdis,max, where thehorizontal axis is the height of the solid-liquid interface, and thevertical axis is G on the crystal side face (the hatched portion is theregion where Vosf,min−Vdis,max >0, and indicates a region wheredefect-free crystals can be produced, while the cross-hatched portionindicates a region that is especially suited to industrial production);

FIG. 10 is a graph of the contour lines for the average value ofVosfmin−Vdis,max, where the horizontal axis is the height of thesolid-liquid interface, and the vertical axis is G on the crystal sideface;

FIG. 11 is a graph of the relation between V/G and the region in whicheach type of defect is present in the radial direction, for the crystalin FIG. 6;

FIG. 12 is a graph of the relation between V/G and the region in whicheach type of defect is present in the radial direction, for the crystalin FIG. 7;

FIG. 13 is a graph of the relation between V/G and the region in whicheach type of defect is present in the radial direction, for the crystalin FIG. 8;

FIG. 14 is a graph of the effect that crystal rotational speed has onthe shape of the solid-liquid interface;

FIG. 15 is a graph of the effect that crucible rotational speed has onthe shape of the solid-liquid interface;

FIG. 16 is a graph of the effect that the strength of the horizontalmagnetic field has on the shape of the solid-liquid interface;

FIG. 17 is a graph of the pull-up rate pattern in the axial direction;

FIG. 18 is an electron data photograph illustrating an X-ray topographof a defect-free crystal;

FIG. 19 is a graph of the regions in which each type of defect ispresent at locations in the lengthwise direction of the crystal and atlocations in the radial direction;

FIG. 20 is a graph of the pull-up rate pattern in the axial direction;

FIG. 21 is a graph of the regions in which each type of defect ispresent at locations in the lengthwise direction of the crystal and atlocations in the radial direction;

FIG. 22 is a graph of the pull-up rate pattern in the axial direction;

FIG. 23 is a graph of the regions in which each type of defect ispresent at locations in the lengthwise direction of the crystal and atlocations in the radial direction;

FIG. 24 is a graph of the relation between oxygen concentration and OSFdensity;

FIG. 25 consists of diagrams illustrating the “shape of the solid-liquidinterface”;

FIG. 26 consists of diagrams illustrating the “height of thesolid-liquid interface”; and

FIG. 27 is a block diagram illustrating the state inside a CZ furnace,giving an example of the means required for implementing the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

As discussed above, defect-free crystals are obtained by controlling thesolid-liquid interface height and the temperature gradient on thecrystal side face to the proper state. It is well known that thetemperature gradient on the crystal side face can be adjusted byadjusting the radiation environment to which the crystal side face aresubjected. As to the adjustment of the solid-liquid interface height,however, a numerical evaluation of the situation is difficult.Nevertheless, the factors that control this, and the qualitativetendencies of the action thereof, are clear, and the right controlconditions can be found by a certain amount of trial and error.

Examples of controlling the height of the solid-liquid interface willnow be given. FIGS. 14, 15, and 16 show how the height of thesolid-liquid interface is related to the crystal rotational speed perunit of time, the crucible rotational speed per unit of time, and thestrength of the applied lateral magnetic field, respectively. It can beseen from the drawings that the more the crystal rotational speed perunit of time is increased, the more the crucible rotational speed perunit of time is increased, and the more the strength of the appliedlateral magnetic field is increased, the more the solid-liquid interfacerises.

The height of the solid-liquid interface is believed to be determined bya thermal balance equation (Formula 2 below) called Stefan's condition.LρV=KlGl−KsGs  (2)

where L is the latent heat of solidification, ρ is the density, V is thegrowth rate, Kl and Ks are the thermal conductivity of the melt and thecrystals, and Gl and Gs are the temperature gradient of the melt and thecrystals.

At the solid-liquid interface, the above equation must always besatisfied for the energy balance, and it is believed that thesolid-liquid interface always moves to the location where the aboveequation is satisfied. In other words, it can be seen that thetemperature gradient Gl of the melt strongly governs the location of thesolid-liquid interface. The temperature distribution within the melt isgreatly affected by the melt convection, so the location of thesolid-liquid interface can be controlled by manipulating the factorsthat govern convection.

An increase in the crystal rotational speed induces a melt hoisting flowthrough crystal rotation, so a high-temperature melt approaches thesolid-liquid interface of the crystal and increases Gl, and thereforeleads to a rise in the solid-liquid interface location. An increase incrucible rotation is known to have the effect of suppressing naturalconvection within the crucible, and augments the effect of theconvection induced relatively by crystal rotation, and therefore leadsto a higher interface. Application of a magnetic field suppressesconvection within the melt and therefore suppresses heat transferthrough convection. Accordingly, the temperature gradient GL within themelt increases, leading to a higher solid-liquid interface. The above isdescribed very well qualitatively. Quantitative prediction requires asimulation of the melt convection including turbulence, and entails somedifficulty, but the qualitative tendency is clear and can therefore becontrolled by a certain amount of trial and error.

[Standard Procedure for Setting Growth Conditions for Defect-FreeCrystals According to the Present Invention]

The standard procedure for setting the conditions for growing thedefect-free crystals according to the present invention is given below.

Step 1: First, referring to FIGS. 9 and 10, the desired growth rate andthe target for the permissible range of growth rate (ΔV) are determined.

Step 2: The hot zone structure with which the set G on the crystal sideface is determined by global heat transfer analysis.

Step 3: A crystal is pulled up in the hot zone determined in Step 2,while the pull-up rate is gradually reduced.

Step 4: The pulled crystal is sliced vertically, and the distribution ofdefect type is evaluated. The pull-up rate at which a neutral region iscreated, and the height of the solid-liquid interface near this pull-uprate are also evaluated.

Step 5: The evaluation results from Step 4 are referenced to FIG. 9, ameans for adjusting the height of the solid-liquid interface and G onthe crystal side face is worked out, and Step 3 is conducted once more.

As discussed above, examples of means for adjusting the height of thesolid-liquid interface include adjusting the rotational speed of thecrucible per unit of time, adjusting the rotational speed of the crystalper unit of time, and applying a magnetic field of a specific strength.The temperature gradient on the crystal side face can be adjusted byadjusting the radiation environment to which the crystal side face aresubjected. The growth conditions under which defect-free crystals willbe obtained can be easily found by manipulation such as this. Insearching for the right growth conditions, it should go without sayingthat design modifications can be worked in as needed by a person skilledin the art.

[Embodiments]

Embodiments of the present invention will now be given.

With the pulling conditions set at a crystal diameter of 210 mm and acrucible diameter of 22 inches, the change in the distribution of defecttype was examined by combination of conditions of solid-liquid interfaceheight and G on the crystal side face. The distribution of defect typeat various locations in the crystal was evaluated by copper decorationmethod and X-ray topography following heat treatment and Secco etching.The results for three different growth conditions are given below asexamples.

TABLE 2 Solid-liquid Temperature gradient Magnetic field Sampleinterface height on crystal side face application Crystal in −5.7 mm2.1° C./mm none FIG. 6 Crystal in +7.0 mm 2.7° C./mm horizontal magneticFIG. 7 field 2000 Gauss Crystal in +14.7 mm  2.82° C./mm  horizontalmagnetic FIG. 8 field 3000 Gauss

The oxygen concentration was from 24 to 32 ppma (old ASTM, F 121-79).FIGS. 6, 7, and 8 illustrate the regions in which each type of defect ispresent, where the horizontal axis is the growth rate, and the verticalaxis is the location in the radial direction of the crystal.

The growth rate range in which defect-free crystals can be obtained isbetween the minimum rate Vosf,min in the radial direction of thetransition rate between the OSF region and the defect-free region, andthe maximum rate Vdis,max in the radial direction of the transition ratebetween the defect-free region and the dislocation cluster region. IfΔV=Vosf,min−Vdis,max, then defect-free crystals will be obtained onlywhen ΔV is positive. ΔV is also the permissible range for pull-up rate,and the larger this value is, the better suited it will be to industrialproduction, and the larger is the average value V of Vosf,min andVdis,max, the better is the productivity.

TABLE 3 Sample Vosf, min Vdis, max V Crystal in FIG. 6 0.320 mm/min0.317 mm/min 0.003 mm/min Crystal in FIG. 7 0.462 mm/min 0.460 mm/min0.002 mm/min Crystal in FIG. 8 0.482 mm/min 0.452 mm/min 0.030 mm/min

The combined results of these experiments are given in FIGS. 9 and 10.FIG. 9 shows contour lines for ΔV, where the horizontal axis is theheight of the solid-liquid interface, and the vertical axis is G on thecrystal side face. The growth of defect-free crystals is possible onlywithin the range indicated by the hatching in FIG. 9, and as for theindustrially controllable range, the range indicated by thecross-hatching in FIG. 9 (ΔV>0.01 mm/min) is preferable.

Meanwhile, FIG. 10 shows the contour lines for the average value V ofVosf,min and Vdis,max, where the horizontal axis is the height of thesolid-liquid interface, and the vertical axis is G on the crystal sideface. It can be seen from FIGS. 9 and 10 that the higher the location ofthe solid-liquid interface, and the higher the temperature gradient, thefaster the defect-free crystals can be pulled up.

When the hatched portion in FIG. 9 is a combination of the height of thesolid-liquid interface and the temperature gradient on the crystal sideface, ^([2]) defect-free crystals can be obtained by pulling at thegrowth rate shown in FIG. 10.

As shown in FIGS. 14, 15, and 16, examples of means for adjusting theheight of the solid-liquid interface include adjusting the rotationalspeed of the crystal per unit of time, adjusting the rotational speed ofthe crucible per unit of time, and adjusting the strength of themagnetic field. As shown in the drawings, the higher the crystalrotational speed is raised, and the higher the crucible rotational speedis raised, and the higher the strength of the horizontally appliedmagnetic field is raised, the higher the solid-liquid interface will be.

It can be seen from FIGS. 9 and 10 that when the height of thesolid-liquid interface is 10 mm or greater, there is a range at whichdefect-free crystals can be produced at a high pull-up rate and over awide permissible range for growth rate. Also, it can be seen from FIG.16 that this solid-liquid interface height can be easily obtained byapplying a horizontal magnetic field of 2500 Gauss or greater.

There are many other factors that affect solid-liquid interface height,all of which can be used as control parameters. For instance, thesolid-liquid interface height rises when the power is increased to theheater installed at the bottom of the crucible. Also, even if the typeof magnetic field is a cusp field, the height of the solid-liquidinterface can be controlled just as with a horizontal magnetic field(lateral magnetic field). The height of the solid-liquid interface canalso be adjusted by immersing in the crucible a second crucible ofsmaller diameter than the crucible or a cylindrical quartz tube. Inother words, any factor that affects the convection of the melt can beused as a means for adjusting the height of the solid-liquid interface.

The temperature gradient on the crystal side face can be adjusted byadjusting the radiation environment to which the crystal side face aresubjected. In other words, the temperature gradient on the crystal sideface can be adjusted as desired through the design of the hot zone,including adjusting the distance between the silicon melt 13 and thebottom of the heat shield 23, and installing a heater or cooler as theside face temperature adjusting means 24.

The height of the solid-liquid interface and the temperature gradient onthe crystal side face vary gradually as the length of the crystalincreases. Therefore, it is preferable to correct and adjust the growthrate slightly as the length increases.

As mentioned above, the temperature distribution within the crystal isdetermined by the “shape of the solid-liquid interface” and the“temperature distribution on the crystal side face.” The inventorsstandardized the “shape of the solid-liquid interface” as the concept ofthe “height of the solid-liquid interface at the crystal center.” Putanother way, the inventors expressed the “shape of the solid-liquidinterface” in representative fashion by the “height of the solid-liquidinterface at the crystal center.” Also, the inventors standardized the“temperature distribution on the crystal side face” as the concept ofthe “temperature gradient on the crystal side face.” Put another way,the inventors expressed the “temperature distribution on the crystalside face” in representative fashion by the “temperature gradient on thecrystal side face.” As discussed above, the inventors indicated that adefect-free region can be easily obtained by setting the conditionsusing these as parameters.

However, the shape of the solid-liquid interface and the temperaturedistribution on the crystal side face can come in a variety of types.Here again, though, the tendencies shown in FIGS. 9 and 10 aremaintained, with only a slight amount of deviation occurring.Specifically, there is no change at all in the fact that the conditionsfor producing defect-free crystals can be readily found by the followingprocedure.

Step 1: First, referring to FIGS. 9 and 10, the desired growth rate andthe target for the permissible range of growth rate (ΔV) are determined.

Step 2: The hot zone structure with which the set G on the crystal sideface is determined by global heat transfer analysis.

Step 3: A crystal is pulled up in the hot zone determined in Step 2,while the pull-up rate is gradually reduced.

Step 4: The pulled crystal is sliced vertically, and the distribution ofdefect type is evaluated. The pull-up rate at which a neutral region iscreated, and the height of the solid-liquid interface near this pull-uprate are also evaluated.

Step 5: The evaluation results from Step 4 are referenced to FIG. 9, ameans for adjusting the height of the solid-liquid interface and G onthe crystal side face is worked out, and Step 3 is conducted once more.

As discussed above, examples of the means for adjusting the height ofthe solid-liquid interface include means for adjusting the rotationalspeed of the crystals, adjusting the rotational speed of the crucible,and adjusting the applied strength of the magnetic field. Thetemperature gradient on the crystal side face can be also be adjusted byadjusting the radiation environment to which the crystal side face aresubjected, as mentioned above.

FIGS. 9 and 10 were drawn for an 8-inch crystal, and therefore cannot bedirectly applied to 6- and 12-inch crystals. However, the process ofusing the adjustment of the solid-liquid interface height and thetemperature gradient on the crystal side face as the process for findingthe conditions under which defect-free crystals can be produced can beapplied directly. Therefore, the conditions under which defect-freecrystals can be produced can be easily ascertained with the presentinvention.

[Production of Defect-Free Ingot]

Embodiments will now be given of the production of a defect-free,slender ingot.

<Example of Crystals Grown without a Magnetic Field>

Crystals were pulled up under conditions in which the crystal diameterwas 210 mm, the crucible inside diameter was 22 inches, the solid-liquidinterface height was −5.7 mm, the temperature gradient on the crystalside face was 2.1° C./mm, no magnetic field was applied, the crystalrotational speed was 12 rpm, and the crucible rotational speed was 12rpm. The pattern of pull-up rate versus pull-up length was as shown inFIG. 17, and the oxygen concentration was about 28 ppma.

FIG. 18 is an X-ray topograph of a sample obtained by slicing the pulledcrystal in the lengthwise direction including the crystal axis, thenperforming a heat treatment for 3 hours at 780° C., plus 16 hours at1000° C. (oxygen atmosphere).

The portions that look white and black in the defect-free sectioncorrespond to a defect-free region where vacancies are dominant and adefect-free region where interstitial silicon is dominant, respectively.Since the amounts of precipitated oxygen are different, a difference incontrast can be seen. However, both regions are free of defects. FIG. 19shows the distribution of defect type as a region diagram. It can beseen from FIG. 19 that a defect-free region was obtained over a widerange of the ingot.

With these growth conditions, in FIG. 9 the permissible growth rate ΔVis located in the positive region, but the ΔV region is extremely small.The production of crystals under these conditions therefore entailsdifficulty.

<Example of Crystals Grown with a Magnetic Field Applied>

Crystals were pulled up under conditions in which the crystal diameterwas 210 mm, the crucible inside diameter was 22 inches, the solid-liquidinterface height was ±13.0 mm, the temperature gradient on the crystalside face was 2.82° C./mm, a horizontal magnetic field of 3000 Gauss wasapplied, the crystal rotational speed was 12 rpm, and the cruciblerotational speed was 1.3 rpm in the opposite direction from the crystalrotation. The pattern of pull-up rate versus pull-up length was as shownin FIG. 20, in which the oxygen concentration was about 26 ppma.

FIG. 21 shows the distribution of defect type. It can be seen from FIG.21 that a defect-free region was obtained over a wide range of theingot. The pull-up rate at which defect-free crystals were obtained was0.43±0.01 mm/min. It can be seen in FIG. 9 that with these growthconditions the permissible growth rate ΔV is positive, and is a largeregion. Actually, it was extremely easy to produce defect-free crystalsunder these conditions.

<Example of Growth at Low Oxygen Concentration>

The oxygen concentration in the crystals in FIGS. 9 and 10 is from 24 to32 ppma (old ASTM, F 121-79), which is currently the practical range foroxygen concentration. However, the inventors discovered that when theoxygen concentration was under 24 ppma, the permissible pull-up rate atwhich defect-free crystals would be obtained in FIG. 9 expanded, and thegrowth rate at which defect-free crystals would be obtained alsoincreased. Here again, the producing conditions can be easilyascertained by using the above-mentioned process for finding conditionsfor producing defect-free crystals. An example of this is given below.

Crystals were pulled up under conditions in which the crystal diameterwas 210 mm, the crucible inside diameter was 22 inches, the solid-liquidinterface height was 14.0 mm, the temperature gradient on the crystalside face was 2.82° C./mm, a horizontal magnetic field of 3000 Gauss wasapplied, the crystal rotational speed was 12 rpm, and the cruciblerotational speed was 1.3 rpm in the same direction as the crystalrotation. The pattern of pull-up rate versus pull-up length was as shownin FIG. 22, and the oxygen concentration was about 12 to 14 ppma.

FIG. 23 shows the distribution of defect type. It can be seen that adefect-free region was obtained over a wide range of the ingot, and thatthe pull-up rate at which defect-free crystals could be obtained washigh over a range of 0.56 to 0.49 mm/min.

There is a report by Iida et al. that lowering the oxygen concentrationraises the rate at which defect-free crystals are generated (M. Lida, W.Kusaki, M. Tamatsuka, E. Lino, M. Kimura, and S. Muraoka (“Defect inSilicon III,” Electrochemical Society Proceedings PV 99-1, 1999, p.499). Also, Sakurada et al. (Japanese Laid-Open Patent ApplicationH11-199386) have shown that when the oxygen concentration is 24 ppma orless, the density at which OSF are generated is extremely low, so it iseasy to produce substantially defect-free crystals.

For example, FIG. 24 shows the relation between OSF density and oxygenconcentration in a heat treatment for 3 hours at 780° C., plus 16 hoursat 1000° C. (oxygen atmosphere). It can be seen from FIG. 24 that no OSFoccur when the oxygen concentration is under 24 ppma. Since no OSFoccur, the pull-up rate range over which a defect-free region isobtained was from Vosf,min to Vdis,max at a high oxygen concentration,but is between the minimum pull-up rate at which void defects occur(Vvoid,min) and Vdis,max, and it can be seen that the range is widerthan under high-oxygen conditions, but specific producing conditionscannot be derived from this finding.

It can therefore be said that the present results cannot be derived fromthese findings, and only with the method of the present invention doesit become possible to find specific producing conditions, which were notdisclosed at all by previous findings. It can also be said that onlywith the method of the present invention is it possible to easily findthe conditions under which an ingot having a defect-free region over awide region can be produced stably and with good reproducibility.

INDUSTRIAL APPLICABILITY

As described above, the present invention makes it possible to producedefect-free crystals stably and with good reproducibility by suitablyadjusting the relation between the shape of the solid-liquid interface,which is the boundary between a silicon melt and a silicon singlecrystal, and the temperature distribution on the side face of a singlecrystal being pulled up, when a silicon single crystal is pulled up froma silicon melt.

With the present invention, regardless of the crystal pulling directionor the wafer plane direction, it is possible to produce an ingot havinga defect-free region over a wide region, stably and with goodreproducibility. Therefore, because a defect-free region can be formedover a wide region in the crystal pulling direction, more defect-freewafers will be taken from a single ingot, so the present invention isfavorable for the mass production of defect-free wafers. Also, because adefect-free region can be formed over a wide region in the wafer planedirection with the present invention, even when a large-diameter ingotis pulled in order to produce a large-diameter wafer (with a diameter of200 mm or more, for example), an ingot in which all of the planes ofthis large-diameter wafer are defect-free can be mass produced stably.

For example, with the present invention, even though a certain amount ofdifficulty may be encountered, it is possible to produce an ingot 55% ofwhich is a region in which all of the wafer planes with a diameter of200 mm are free of defects, even when no magnetic field is applied (FIG.19). Also, when a magnetic field (2500 Gauss or greater) is applied, itwill be easy to produce an ingot 70% or greater of which is a region inwhich all of the wafer planes with a diameter of 200 mm are free ofdefects (FIG. 21).

Furthermore, defect-free crystals with a low oxygen concentration can beproduced stably with the present invention. This is also true in theproduction of a large-diameter wafer with a low oxygen concentration.(See FIG. 23. This allows an ingot 40% or greater of which is a regionin which all of the wafer planes with a diameter of 200 mm to beproduced with ease).

1. A method for producing a silicon ingot by pulling up a silicon singlecrystal from a silicon melt by the Czochralski method, wherein theheight of a solid-liquid interface between a silicon melt and a siliconsingle crystal at a crystal center of the solid-liquid interface isadjusted by one or more ways selected from a group consisting ofadjusting a strength of a magnetic field applied to the silicon melt,adjusting a rotational speed per unit of time of a crucible containingthe silicon melt, and adjusting a rotational speed per unit of time ofthe silicon single crystal, and wherein a temperature distribution on aside face of the silicon single crystal is adjusted, whereby a siliconingot including a defect-free region is produced.
 2. A method forproducing a silicon ingot by pulling up a silicon single crystal from asilicon melt by the Czochralski method, wherein the height of asolid-liquid interface between a silicon melt and a silicon singlecrystal at a crystal center of the solid-liquid interface is adjusted byone or more ways selected from a group consisting of adjusting thestrength of the magnetic field applied to the silicon melt to be 2500Gauss or greater, adjusting a rotational speed per unit of a cruciblecontaining the silicon melt and adjusting a rotational speed per unit oftime of the silicon single crystal, and wherein a temperaturedistribution on a side face of the silicon single crystal is adjusted,whereby a silicon ingot including a defect-free region is produced.
 3. Amethod for producing a silicon ingot by pulling up a silicon singlecrystal from silicon melt by Czochralski method, wherein the height X ofa solid-liquid interface between the silicon melt and the silicon singlecrystal at a crystal center portion of the solid-liquid interface andtemperature gradient on a side face of the silicon single crystal Y areadjusted to be within a range of (1) 1.5° C./mm Y 2.0° C./mm for −20 mmX 0 mm, or (2) 1.5° C./mm Y 0.1X+2° C./mm for 0 mm<X<10 mm, or (3)0.1X±0.5° C./mm Y 20.2X+1° C./mm for 10 mm X 20 mm, and wherein themethod excludes pulling up the silicon single crystal under any of thefollowing single crystal pull-up conditions I to III: (I) a ratioexpressed by V/G is controlled to be in a range from 0.20 to 0.22 mm²/°C.min, where V(mm/min) is a crystal pull-up rate and G (° C.mm) is anaverage temperature gradient within the crystal in an axial directionbetween melting point of silicon and 13000° C.; (II) an in-plane averageG is less than 3° C./mm, less than 1.0° C./mm between G_(edge) andG_(center), V is controlled to be V_(OSFclose) ±0.02 mm/min, average Vis controlled to be V_(OSFclose) ±0.01 mm/min, and a magnetic fieldstrength of 2000 G or greater in a horizontal magnetic field is applied,where G_(edge) is an average axial temperature gradient on the crystalside face side, G_(center) is an average axial temperature gradient onthe crystal center side, and V_(OSFclose) is the pull-up rate at whichOSF rings disappear when the pull-up rate is reduced; and (llI) a V/Gvalue between a crystal center location and a location up to a crystalouter periphery is from 0.16 to 0.18 mm^(2/)° C. min, andG_(outer)/G_(center) is 1.10 or less where V is a crystal pull-up rate(mm/min), G (° C./mm) is an average temperature gradient within thecrystal in an axial direction between the melting point of the siliconand 1350° C., G_(outer) is the G value on an outer side face of thecrystal, and G_(center) is the G value at a crystal center.
 4. Themethod according to claim 3, wherein the silicon single crystal ispulled at a rate of 0.40 mm/min or higher.
 5. The method according toclaim 3, wherein the silicon single crystal has an oxygen concentrationof 24 ppma or less and a diameter of 200 mm or greater and for 40% orgreater (in terms of a ratio of length to overall silicon ingot length)an entire wafer surface is a defect-free region.